Semiconductors are usually characterized as either n-type or p-type, depending upon whether the predominant carriers in the material are electrons or holes. As is known, semiconductors can be rendered n-type or p-type by substituting impurity atoms (dopants) for atoms of the host lattice which have a different valence. Donor-type impurities are those which give electrons, and thus render the host material n-type, while acceptor-type impurities are those which receive electrons, and thus render the host p-type.
Successful doping to obtain or enhance n-type or p-type conductivity depends not only upon the ability to introduce a sufficient amount of the proper dopant into the semiconductor material, but also upon the ability to position the dopant atoms in the proper substitutional sites within the material's crystal lattice where they can give or receive electrons.
Dopants which do not readily assume the proper substitutional sites in sufficient number can be activated or converted to donors or acceptors. For example, they may be activated by a thermal anneal of the doped semiconductor material.
Another important consideration is the presence of other impurities in the semiconductor material which are, or are capable of assuming, an opposite conductivity type than that intended, thus compromising the effect of the dopant. Thus, it is actually the net donor or acceptor concentration which determines the overall conductivity of the material.
Semiconductors which can be rendered either n-type or p-type with the appropriate doping such as Si, a Group IV element, and GaAs, a III-V compound, can be converted to devices such as diodes by doping adjacent regions p- and n-type to form pn junctions.
Group II-VI compounds such as ZnS and ZnSe are of great interest currently for use in semiconductor devices because of their relatively wide band gaps. For example, light-emitting diodes (LEDs) and diode lasers operating in the blue region of the visible spectrum may be formed from doped junctions in epitaxial layers of ZnSe.
In practice, it has proved to be extremely difficult to obtain stable doped ZnSe epitaxial layers. While a sufficient amount of dopant can usually be introduced into the layers, it is either difficult to convert sufficient numbers of the dopants into acceptors incorporated into the lattice of such compounds, or the acceptors are unstable. For example, lithium-doped epitaxial layers of ZnSe can be converted to p-type material, but lithium is unstable because of its tendency to diffuse, even at relatively low temperatures.
Nitrogen has been proposed as a more stable acceptor dopant than lithium, and although it can be doped into ZnSe in-situ in high concentrations, it has been found that only a small fraction of such nitrogen can be activated. Additionally, nitrogen doping by chemical vapor deposition (CVD) is difficult to accomplish because (1) parasitic chemical reactions with the zinc and selenium precursors during growth can keep nitrogen from being incorporated, and (2) nitrogen can be electrically compensated (neutralized) by hydrogen in the material.
To date, epitaxial layers of ZnSe and its alloys have been grown using techniques such as Molecular Beam Epitaxy (MBE), Metal-Organic Molecular Beam Epitaxy (MOMBE), Chemical Beam Epitaxy (CBE) and Metal-Organic Vapor Phase Epitaxy (MOVPE). The first three techniques (MBE, MOMBE and CBE) require the use of an high vacuum environment on the order of 10.sup.-6 Torr. Also, in MBE the constituent elements are supplied to the substrate from elemental sources, or in some cases from alloy compound sources. In MOMBE and CBE the metalorganic compounds and hydrides are supplied in their cracked forms (elements along with the products of thermal decomposition) or compound forms, respectively.
MOVPE is conventionally performed at pressures ranging from 800 to 1 Torr using a combination of metalorganic and hydride compounds as sources. This operational pressure range ensures a laminar flow of the reactants in the vicinity of the substrates, resulting in good uniformity over a large area.
An important difference between MOVPE and the other techniques is that the Mean Free Path (MFP) in the case of the high-vacuum techniques is several orders of magnitude larger than the source to substrate distance. As a result, interaction between the various species occurs only at the surface of the substrate. In the case of MOVPE, the MFP is several orders of magnitude smaller than the source to substrate distance. As a result, the various species can undesirably interact with themselves (such as recombination of excited dopant species) or among each other (such as parasitic reactions between Zn and Se precursors).
Conventionally, MOVPE growth of ZnSe occurs by the reaction of dimethylzinc with dimethylselenide at the substrate surface with temperatures above 500.degree. C. and pressures of about 300 Torr. However, this has not led to sufficient nitrogen incorporation or a sufficient net acceptor concentration, primarily because of the compensation of nitrogen by hydrogen during growth. The possible sources of hydrogen are the carrier gases, the decomposition products of the organometallics, and the decomposition of ammonia when it is used as the source of nitrogen.
The precursor combination of DMZn and H.sub.2 Se has been previously investigated in MOMBE and CBE. See Yoshikawa et al, "Growth Kinetics in MOMBE of ZnSe Using DimethylZinc and HydrogenSelenide As Reactants", J. Cryst. Grow., 94, 69 (1989). This method uses ultra low pressure during growth, requires that the process be performed in ultrahigh vacuum MBE equipment, and does not use nitrogen doping.
Nitrogen plasma generated using a radio frequency (r.f.) plasma source has been used for acceptor doping in MBE. See Park et al, "p-type ZnSe By Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth", Appl. Phys. Lett. 57, 2127 (1990) using MBE with elemental zinc and selenium; and Ohkawa et al, "Characteristics of p-type ZnSe Layers Grown By Molecular Beam Epitaxy With Radical Doping", J. Appl. Phys., 30, L152 (1991) using MBE with elemental Zn and elemental Se as the growth sources grown under high pressures.
Microwave plasma source has also been used to generate a remote plasma using ammonia for nitrogen acceptor doping in conventional (3 Torr) MOVPE. See Huh et al, "Low Pressure MOVPE of ZnSe With Hydrogen Selenide and Dimethylzinc-Triethylamine", J. Electron. Mat. 22. 509, (May 1993)
There continues to be a need to further the ability to produce epitaxial layers of II-VI semiconductor compounds suitable for use in the growth of blue laser devices. The present invention provides such a method and novel epitaxial layers derived therefrom.